Enhanced Post Deposition Annealing Conditions on the Fabrication of High Quality Thermochromic Vanadium Dioxide Films

Abstract

Sputter deposition of thermochromic VO 2 thin films for smart window applications has been faced with several challenges including the need for high deposition temperature, extremely precise and narrow range of oxygen/argon flow ratio and target poisoning during sputtering. Deposition of VO 2 at room temperature without oxygen followed by post-deposition annealing has been cited as one of the potential mitigations to the challenge. In this study, the effects of post- deposition annealing conditions on the structural, electrical and optical properties of VO 2 thin films are reported. The films were prepared on soda lime glass substrates using DC magnetron sputtering of metallic vanadium target in argon atmosphere, at room temperature without oxygen followed by post-deposition annealing. The working pressure and argon flow rate were mbar and 76 ml/min, respectively. Thereafter, the films were annealed in a low vacuum environment at different temperatures and times. The XRD results confirmed that all films annealed at different temperatures and times were monoclinic VO 2 except for films annealed at a temperature of 300 o C. The results indicate that 400 o C and 20 minutes are the optimum annealing temperature and time, respectively, for fabrication of high-quality VO 2 films with improved stoichiometry and crystallinity. The films had resistivity change between the semiconducting and metallic phases of one order of magnitude and the transition temperature between 58 o C and 63 o C. The highest peak transmittance and best optical switching of the annealed VO 2 films was 53% and 18%, respectively, at a wavelength of 2500 nm for film thickness of 44 nm. The results suggest that room temperature deposition without oxygen followed by post-deposition annealing in a low vacuum environment may be adequate for the fabrication of switching VO 2 thin films.